Codeforces Beta Round #35 (Div. 2) and Codeforces Beta Round #36 <p> Greetings to everybody. </p> <p> Today is an unusual day on Codeforces: two contests a day with quite small gap between them. The reason is that today the IX Regional team school programming contest is held in Saratov, and it was decided to use the tasks from it for Codeforces rounds. </p> <p> That's why there are many authors of today's tasks. I would like to thank the whole jury of the school contest for their excellent job. The following people are on the jury: <font color="#0000ff"><u><a href="http://codeforces.me/profile/RAD"><span lang="en-US">Artem Rakhov</span></a></u></font><span lang="en-US">, </span><font color="#0000ff"><u><a href="http://codeforces.me/profile/NALP"><span lang="en-US">Nickolay Kuznetsov</span></a></u></font><span lang="en-US">, </span><font color="#0000ff"><u><a href="http://codeforces.me/profile/natalia"><span lang="en-US">Natalia Bondarenko</span></a></u></font><span lang="en-US">, </span><font color="#0000ff"><u><a href="http://codef...

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Codeforces Round #983 (Div. 2) ### Hello Codeforces! $\newline$ <spoiler summary="From AkiLotus"> > "<span style="color: red">K</span>aleidoscopic, this place still is. How long have I been away...?" Hi. Akik<span style="color: red">a</span>ze a.k.a. AkiLotus here. Four years have passed, lo<span style="color: red">n</span>g enou<span style="color: red">g</span>h huh? I h<span style="color: red">o</span>pe that after all these times, I am not too rusty to set good tas<span style="color: red">k</span>s for yo<span style="color: red">u</span> to enjoy, starting with <span style="color: red">t</span>his <span style="color: red">o</span>ne! <strong>See yo<span style="color: red">u</span> in the round!</strong> <span style="color: #ec3e9d"><i>"Waeeguyehy" is coming soon.</i></span> </spoiler> It's been a while, today [user:AkiLotus,2024-07-29] and I are delighted to invite you to participate in [contest:2032]. This round will be <strong>rated</strong> for all participants with a rating lower than <...

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Contests for Math: Repovive Math Contest Grand Opening Hello everyone! Have you ever wished that math and combinatorics problems had the same live contest experience as competitive programming? We have, so we are building exactly that on Repovive! After beta testing our new Math section, we are ready to host the **Repovive Math Contest Grand Opening**. You can already try our beta math problems here: https://repovive.com/problems/math The theme of our first contest will be **Combinatorics**. The round will contain **5 combinatorics problems** to be solved in **2 hours and 30 minutes**. The problem set is intended to be suitable for a wide range of levels, so everyone is welcome to participate. The contest is **rated for everyone**. Since this is the first large live test of this format, we would especially love to have you participate, try the judging system, and share your feedback afterward. * Start Time: [Tuesday, 29 September 2026, 14:35 (UTC)](https://www.timeanddate.com/worldclock/fixedtime.html?iso=20260929T1...

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Codeforces Beta Round #92 <p>&quot;Tryam&quot; to everyone!<br /><br />I am an authors of the today's contest. First of all I want to thank <b>Artem Rakhov</b> [[[user:RAD,2011-11-03]]] (thanks to him, you'll understand my problems) and <b>Maria Belova</b> [[[user:Delinur,2011-11-03]]] (those who know my English level will understand a huge of my thanks). Also thanks to everyone staff for the wonderful of the codeforces system. And also thanks to <b>Sergei Tarasov</b> [[[user:Seryi,2011-11-03]]] and <b>Andrey Tkachenko</b> [[[user:TBGame,2011-11-03]]] for ideas generating and testing.<br /><br />Good luck and have fun.</p> <p><span lang="en"><span class="hps">Today,</span> <span class="hps">a breakdown of</span> </span>score <span lang="en"><span class="hps">will be slightly different</span> <span class="hps">from the standard</span> <span class="hps">-</span> <span class="hps">for</span> <span class="hps">the second division</span> <span class="hps">is <b>500-1000-2000-2000-2500</b></span> <span class="hps"...

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Hackers ranking (beta) ### FAQ **What is this?** Have you ever wondered who is the best hacker on Codeforces? Here is simple solution, which would try to answer for your problem! I would like to introduce new system of grading hackers in rounds. [cut] <p></p> <b>How does it work?</b> The whole rating system is based on <a href="http://en.wikipedia.org/wiki/Elo_rating">Elo rating</a>. The problem is how to measure user's performance in each round. **Yes, how?** The whole system is now in phase of testing, so some solutions may be changed (and probably will be changed :)). So let's consider some user during some round. Let's call him Bob. **I want to choose another name, may I?** No. **Ok.** So, let's say that Bob solved three problems: A, B and C (Bob is pretty sad), but also hacked one solution for B and had one unsuccessful hack attempt. Bob doesn't know that his solution for C won't pass system tests. How would we know if it was good or bad? At first, we will look at al...

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Complex FFT is not as bad as you think ... Probably. Hi everyone! As some of you _might_ already know, I don't like NTT. Primary reasons for this are: - I don't like modular arithmetic optimizations (Montgomery, etc). - I don't like NTT mods, and prefer good, old $10^9+7$. - I do like algebra of complex numbers. But, to my regret, the overwhelming mainstream in modern competitive programming is NTT. Primary reasons are: - It's allegedly faster. - It needs less memory. - It has no precision issues. - People don't like algebra of complex numbers. There is nothing I can do about the last, but today I'll address the first 3. Optimizing complex FFT was on my mind for quite some time, but I didn't really work on it _that much_ until the [blog](https://codeforces.me/blog/entry/142063) by [user:Qwerty1232,2025-04-24] on optimizing NTT has dropped, which motivated me to actually put some serious effort into this. In this blog, we will primarily focus on optimizations that are specific to complex arithmetic...

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Solutions to Codeforces Beta Round #35, A, B, C, D A. Shell Game The solution is straightforward by swapping the integers as required by the problem. B. Warehouse The warehouse can be viewed as a two-dimensional array. If '+1' is met, we just enumerate the positions from the given one to the final one in the order required by the problem, and put the drink into the first empty box if there exist any. If '-1' is met, we can enumerate from the first position to the final one, and if any one that has the same ID is found, we just output the corresponding position; otherwise output "-1 -1". C. Fire Again Suppose that the position at which the fire begins is denoted as (f1,f2). Then, for any position (r,c), the fire gets to it after time |r-f1|+|c-f2|, which is referred to as Manhattan Distance as far as I consider. Therefore, we can first enumerate the positions where fire starts, and for each such position, we enumerate every feasible position and calculate the time when the fire arrives at it. As all the fires start at the...

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On the mathematics behind rolling hashes and anti-hash tests This blog assumes the reader is familiar with the basic concept of rolling hashes. There are some math-heavy parts, but one can get most of the ideas without understanding every detail. The main focus of this blog is on how to choose the rolling-hash parameters to avoid getting hacked and on how to hack codes with poorly chosen parameters. # Designing hard-to-hack rolling hashes ## Recap on rolling hashes and collisions Recall that a rolling hash has two parameters $(p, a)$ where $p$ is the modulo and $0 \leq a < p$ the base. (We'll see that $p$ should be a big prime and $a$ larger than the size $\left|\Sigma\right|$ of the alphabet.) The hash value of a string $S = s_0 \dots s_{n-1}$ is given by $$ h(S) := \left(\sum\limits_{i=0}^{n-1} a^{n-1-i} s_i\right) \mod p $$ For now, lets consider the simple problem of: given two strings $S, T$ of equal length, decide whether they're equal by comparing their hash values $h(S), h(T)$. Our algorithm declares $S$ and $T$ t...

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Blog collection <p>________________________________________________________________________________________<br><br></p><h3><b>Puzzle</b></h3><ul><li><a href="http://codeforces.me/blog/entry/66">Red and Black hair</a>, </li><li><a href="http://codeforces.me/blog/entry/112">The 12 chairs and diamonds</a></li><li><a href="http://codeforces.me/blog/entry/228">Monty Hall Problem</a></li></ul><br><br><h3>Useful <br></h3><ul><li><a href="http://codeforces.me/blog/entry/70">Petr Mitrichev, 3 days in Sao Paulo Training Camp 2010</a></li><li><a href="http://codeforces.me/blog/entry/512">Useful links</a></li><li><a href="http://codeforces.me/blog/entry/529">Problem Set Analysis Of CodeForces</a></li><li><a href="http://codeforces.me/blog/entry/546">Discussions Of CodeForces Round</a></li><li><a href="http://codeforces.me/blog/entry/1492">ALL CodeForces rounds Tutorial</a></li><li><a href="http://codeforces.me/blog/entry/1534">Online Judge Locator</a></li><li><a href="http://codeforces.me/blog/entry/170...

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Solutions to Codeforces Beta Round #44, A, B, C, D(Branch and Bound) A. Triangular numbers An integer n is a triangular number if we can find such an integer m that n=m*(m+1)/2. As n is not larger than 500, we can enumerate all integers from 1 to 1000 (not necessarily 1000, for instance 500 is also enough) and check whether we can find such an integer m or not. B. Coins If a team T wins against another one, we can add one point for team T. Thus, for team A, B and C, if they have points with forms of {0,1,2}, the results are not contradictory; otherwise this is impossible. C. Crossword A little bit complicated problem for me.. As there are only 6 words, we can enumerate all the feasible 6!=720 patterns and check if they can form a reasonable crossword puzzle. Without loss of generality, we denote the 6 strings as s[6], and further assume that s[0], s[2], s[4] are parallelly placed while the other three strings are vertically placed. Thus, a reasonable crossword puzzle should satisfy the following several conditions: 1) The total len...

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Codeforces in the very beginning of 2022 On January 4th I ran [my script] (https://github.com/NataIss/codeforces) to download user profiles and sum how many users registered in different years had already logged to CF in 2022. Today is 9th, but I have some problem with API and could not update my data. So if somebody had been online already but was not included in statistics and doesn't like this fact &mdash; feel free to write DM to me or add a comment and I'll fix the issue :-) Pay attention that bar chats use different scales.  [cut] _._ If you open [dashboard](https://public.tableau.com/app/profile/natalia5014/viz/Codeforcesintheverybeginningof2022/Dashboard1?publish=yes) directly you could expand Year to Quarters, Quarters to Month etc clicking on [+] on column header.You can select View data in left-click menu for every column and then select Full data and discover user profiles in details. And even find for example all people registe...

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Chess Engine Algorithms — A Complete Tour, From Bitboards to NNUE Chess engines are one of the best case studies in competitive programming for how search and heuristics combine to solve a problem that's too big to brute-force. A chess position has a branching factor of roughly 35, and a full game lasts ~40–80 moves per side — the game tree is astronomically larger than what any computer could ever fully explore. Everything an engine does is about making a huge search space tractable. This post walks through the algorithms modern engines are built from, roughly in the order you'd add them if you were building one yourself: representation → move generation → search → evaluation → modern neural approaches. 1. Board Representation Before you can search anything, you need a fast way to represent the board and generate moves. 8x8 array The simplest approach — a char board[8][8]. Easy to reason about, but slow: move generation and attack detection require scanning rows/columns/diagonals cell by cell. Bitboards Almost every serious en...

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October Schedule <img src="http://assets.codeforces.com/images/oct2010.png" style="float:left;width:150px;margin:1em 2em 1em 1em;"> Pay attention that on the "Contests" page the October schedule is available. Key dates are: <div><ul> <li>Oct. 7&nbsp;- Codeforces Beta Round #33</li> <li>Oct. 11&nbsp;- Codeforces Beta Round #34 (Div. 2)</li> <li>Oct. 19&nbsp;- Codeforces Beta Round #35 (Div. 2) and Codeforces Beta Round #36</li> <li>Oct. 25&nbsp;- Codeforces Beta Round #37</li> <li>Oct. 30&nbsp;-&nbsp;Codeforces Beta Round #38 (ACM-ICPC Rules)</li> </ul> <div>Now from the contests page you can go directly to the link to &nbsp;<a href="http://timeanddate.com/">timeanddate.com</a>&nbsp;to see the beginning of a contest in your time zone </div></div><div><br></div> <div>Look for updates&nbsp;- it can be changed and widen.</div><div><br></div> <div>Wish you high rating,</div><div>MikeMirzayanov and the Codeforces team</div>

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Topics Problems Good Day to you! While a lot of us, when he begins to start competitive programming he found difficult to found problems on some topics in the beginning to practice of it, and same thing for a lot of ICPC Communites that they have started. These are some topic not advanced and videos, problems and articles on them. <br> <spoiler summary="Prefix Sum & Frequency Array"> [Wonderful Coloring](https://codeforces.me/contest/1551/problem/B1) [Do Not Be Distracted](https://codeforces.me/contest/1520/problem/A) [Letter](https://codeforces.me/problemset/problem/43/B) [Pangram](https://codeforces.me/problemset/problem/520/A) [Andryusha and Socks](https://codeforces.me/contest/782/problem/A) [Count Numbers](https://codeforces.me/group/c3FDl9EUi9/contest/262795/problem/A) [Count Characters](https://codeforces.me/group/c3FDl9EUi9/contest/262795/problem/B) [Range Sum Query](https://codeforces.me/group/c3FDl9EUi9/contest/262795/problem/E) [Count a's](https...

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ALL CodeForces Rounds Tutorial <html> <body> <br /><br /> >>>>>>>**Hi I just Say Really Sorry For Delay in UPD This Blog...!**<br /><br /><br /> <u>This is not Complete</u> , but i try to find <b>all Tutorial codeforces round</b> and posted here , thanks in advance :D <br /><br />This post will <b> <span class="Apple-style-span" style="border-collapse: collapse;color: rgb(255,0,0);font-family: verdana , arial , sans-serif;font-size: 12.0px;"> UPD</span></span></b> after each contest . <span>If anyone have or writes any editorial that's</span> <b> <span> <span class="Apple-style-span" style="border-collapse: collapse;color: rgb(255,0,0);font-family: verdana , arial , sans-serif;font-size: 12.0px;"> not in here</span></span></b> , Plz post here for ALL . <br /><br /><b>UPD </b> &nbsp;: Table1 updated : in Future!!! <br /><br /><b>UPD </b>&nbsp;: Table2 updated : CF # 118 ...to... CF # 169 added .<br /><br /> </p><hr /><br /></p> <div align="justify"> <br /><br /><b><s...

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Need clarification for the problem “ Need For Brake ” (Codeforces Beta Round #66 Problem B ) <p class="MsoNormal">The problem link&nbsp;<a href="http://www.codeforces.com/contest/73/problem/B">http://www.codeforces.com/contest/73/problem/B</a><br /></p><p class="MsoNormal">Consider this sample input:</p> <p class="MsoNormal">3</p><p class="MsoNormal">teama 1</p><p class="MsoNormal">teamb 20</p><p class="MsoNormal">teamc 10</p><p class="MsoNormal">3</p><p class="MsoNormal">17 29 35</p><p class="MsoNormal">teama</p><p> </p><p class="MsoNormal">In the custom test I ran <span class="apple-style-span"><span style="font-size: 10.0pt;line-height: 115.0%;font-family: Verdana , sans-serif;color: black;"><b><span style="font-family: Arial , sans-serif;color: rgb(0,0,204);"><a href="http://www.codeforces.com/contest/73/submission/374251">sohelH</a></span></b></span><a href="http://www.codeforces.com/contest/73/submission/374251"><b>&rsquo;s code</b></a> for the above input which was Accepted in the contest. It gives 3 3 as output.</span></p> <p class="MsoNormal"><span class="apple-sty...

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Codeforces Beta Round #35 (Div. 2) by http://www.acmwiki.com A 水题。模拟。 B 水题，唯一的技巧，把两维的描述写成一维的就简洁了。 关键代码如下： int N,M,K; string a[MAXN]; char name[MAXN]; int main() { freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout); scanf("%d%d%d",&N,&M,&K); REP(i,MAXN) a[i]=""; REP(i,K) { char cs[10]; scanf("%s",cs); if(cs[0]=='+') { int x,y;scanf("%d%d%s",&x,&y,name);x--,y--; rep(j,x*M+y,MAXN) if(a[j]=="") { a[j]=string(name);break; } } else { int x=-1,y=-1;scanf("%s",name);string s=string(name); REP(j,N*M) { if(a[j]==s) { a[j]="",x=j/M+1,y=j%M+1; break; } } printf("%d %d\n",x,y); } } return 0; } C 水题，可以直接算，也可以floodfill。 D 水题。dp和贪心均可。 E by http://www.acmwiki...

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Scaler Academy: Our side of the story: AMA Hello folks, My name is Anshuman and I am the co-founder of Scaler Academy and InterviewBit. I am a competitive coder myself, since 2006, been a part of Codeforces since 2009, when Beta rounds were happening. Been to ACM ICPC World Finals twice. Ex-Facebook where I was one of tech leads on the messages and messenger team. I believe every company/program gets criticism and it should only be taken constructively. While most of the blogs here on Scaler Academy have been very far from truth, I never came to respond here because it’s hard to reason with anonymous fake accounts. However, the past few days, these folks have started harassing and abusing current Scaler Academy students and mentors. They created a phishing website clone of ours to steal student credentials. They have bullied anyone who has tried to express their opinion if it was positive towards scaler. **That, in my dictionary, is online bullying and very unethical.** Attaching images of students and mentors wh...

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Два контеста codeforces в один день?? <p>Codeforces Beta Round #35 (Div. 2) 19.10.2010 11:00<br></p><p>Codeforces Beta Round #36              19.10.2010 14:00<br></p><p>Два разных контеста в течении 5 часов? (будет интересно)</p>

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Поправочные коэффициенты и инфляция рейтинга <P>Результат последнего контеста оказался для меня довольно неожиданным, поэтому я решил собрать немного больше информации о возможностях изменения рейтинга CF. Если профиль участника позволяет наблюдать историю изменения&nbsp;его рейтинга, то у меня получились "профили контестов", в которых отражены рост или падение каждого участника конкретного соревнования.<BR>[cut]<BR><A href="http://codeforces.me/blog/entry/102">Формула расчёта&nbsp;рейтинга Codeforces</A> подразумевает сохранение суммы очков всех участников до и после раунда за счёт поправочных коэффициентов. На основании же обработанной мной информации получилась такая табличка:</P> <TABLE style="WIDTH: 100%" border=1> <TBODY> <COLGROUP> <COL width="20%"> <COL width="20%"> <COL width="20%"> <COL width="20%"> <COL width="20%"> <TBODY> <TR> <TD>ID контеста</TD> <TD>Суммарный рейтинг до соревнования</TD> <TD>Суммарный рейтинг после соревнования</TD> <TD>Разность конечного и исходного суммарных рейтингов</TD> <TD>Коли...

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